The scattering theory analysis of [1]} and Section  show that
\(\sigma _{\rm ess}(H^\sharp (k_x,a))=\sigma (H(k_x)) =\; ]-\infty ,-\sqrt{k_x^2+(f-\nu k_x^2)^2}]\,\cup \,\lbrace 0\rbrace \,\cup \,[\sqrt{k_x^2+(f-\nu k_x^2)^2},\infty [.\) 
